Add helper function to install headers
Add helper function to install headers

Add an additional helper function lcm_install_headers for installing
C/C++ headers. Very much like lcm_install_python, the main advantage to
this is that it preserves the package name subdirectory components.
(Internally, the two functions share about half their logic, with only
the default destination differing.)

lcm-proj/lcm,adeschamps/lcm,bluesquall/lcm,lcm-proj/lcm,bluesquall/lcm,lcm-proj/lcm,adeschamps/lcm,bluesquall/lcm,adeschamps/lcm,bluesquall/lcm,adeschamps/lcm,lcm-proj/lcm,lcm-proj/lcm,bluesquall/lcm,adeschamps/lcm,lcm-proj/lcm,bluesquall/lcm,adeschamps/lcm,lcm-proj/lcm,lcm-proj/lcm,adeschamps/lcm,adeschamps/lcm,bluesquall/lcm